1. Field of the Invention
This invention relates to the fabrication and use of flexure assemblies to support slider mounted read/write transducers in disk drives. More particularly, it relates to a flexure design that can be used advantageously in near-contact recording where the slider-disk interference (HDI) is intense.
2. Description of the Related Art
A hard disk drive (HDD) uses an encapsulated thin film magnetic read/write head (transducer), called a slider, to read and write data on a magnetic medium or storage disk. The slider has a pre-patterned air-bearing surface (ABS) and is mounted on a head gimbal assembly (HGA). The HGA is activated by a servo actuator and associated electronic control circuitry to position the slider at various target locations along the magnetically encoded tracks on the disk. As the disk is rapidly rotated by a spindle motor, hydrodynamic pressure causes an air flow between the ABS of the slider and the surface of the disk. This flow lifts the slider so that it literally flies above the surface of the disk (at a “fly height”) on a layer of air. The edge of the slider into which the disk rotates is called its “leading edge,” the opposite edge, which contains the read/write head is called the “trailing edge.” The HGA also includes a flexible connection or flexure, between the slider and a load beam (suspension), allowing the slider pitch and roll capability when fly height is achieved. The flexure maintains the ABS of the slider substantially parallel to the surface of the disk. Early forms of this flexible suspension are disclosed in Levy et al. (U.S. Pat. No. 4,797,763), who, in the abstract, describes the slider as being “secured to the mounting arm by a small springy resilient sheet metal member, having a broad central area with a rounded protrusion intended to engage the outer transverse portion of the ring-shaped end of the mounting arm . . . .” The flexure thus enables the slider to pivot about a dimple (Levy's “rounded protrusion”) on the load beam, with relatively low resistance. Other early disclosures of flexible slider mountings are found in Villette (U.S. Pat. No. 4,280,156) and King (U.S. Pat. No. 4,399,476).
Thus, thanks to developments in the gimbals and flexure, even despite manufacturing tolerances, the parallel positioning of the slider can be maintained. A more recent slider and HGA combination of this type is described in Khan et al. (U.S. Pat. No. 5,115,363). Khan teaches a gimbal-type flexure having a lateral cross band support member for flexibly coupling a slider mounted transducer to a load beam on the actuator arm of a hard disk drive. A novel feature of Khan et al. is that the slider mounts to the flexure at its mid-section rather than distally.
For discussion purposes, the angle between the slider ABS and the disk surface in a cross-section along the slider axis of symmetry, is called the pitch angle. The angle between the ABS and the disk surface in a cross-sectional plane perpendicular to the slider axis of symmetry is called the roll angle. It is known in the prior art that the slider fly height is affected by moments in pitch and roll directions. It is also known that high flexure stiffness in the radial direction (transverse to the data tracks the disk) is beneficial to maintaining accuracy of read/write head positioning during the accessing of a target track. Such stiffness requirements have been met by the so-called Watrous flexure (described in U.S. Pat. No. 5,568,332), on which the slider is mounted distally and which is widely used in the industry. If the sliders are to be repeatedly lifted within the disk drive during drive operation, the flexure must also provide a suitable pitch moment during lift. A “ring-gimbal” type flexure has been adopted for this purpose and is described in Villette, cited above.
A pre-load downward force applied by the suspension is used to counteract and control the hydrodynamic lifting force. The position above the disk at which the pre-load downward force and the hydrodynamic upward force are in equilibrium is the targeted fly height of the slider. When a predetermined rotational speed and targeted flying height are achieved, the writing and reading of data commences. As a consequence of higher linear and track densities, the flying height and thus the distance between the read/write head and the storage disk, must be extremely small to ensure accurate transfer of data.
It is known in the prior art that the flexure affects slider fly height modulation by affecting the various modes of HGA vibration. In particular, prior art attention has been focused on low frequency vibrational modes of the suspension, typically frequencies below 10 kHz, including the sway mode, the twist mode, and the bending mode. These modes are typically excited by turbulence in the air flow about the slider, as well as by the operation of the servo actuator and spindle motor bearing. Khan et al. (U.S. Pat. No. 5,115,363), already discussed above, discloses an HGA and flexure that has greatly increased compliance for pitch and roll of the slider and yet does not permit the mass of the slider to generate a moment about its mounting on the flexure. The flexure provided has a generally fork-shaped outer member with a low stiffness cross-bar formed between the outer prongs of the fork. The slider is mounted on the cross-bar. The primary vibrational modes of the mounted slider are in the 400 Hz to 6 kHz range, indicating a high degree of tracking stability. Ueda et al. (U.S. Pat. No. 5,630,948) primarily teaches a method of forming an integrated HGA and suspension wherein pairs of signal carrying conducting lines are integrated within the entire structure. Ueda's flexure is H-shaped and, being over-constrained, is inherently stiff. The method of Ueda is indicated as being advantageous for carrying “any suitable head by any desired means” (column 3, lines 15-16), but the flexure design indicated in the associated figure is significantly different from that shown in Khan et al. in that Khan's flexure is M-shaped. In addition, in Ueda there is neither instruction nor analysis on the optimal location of crossbars connecting the slider to slender, long, flexible strips. Erpelding et al. (U.S. Pat. No. 6,351,348) teaches a suspension system in which the positioning of the conducting leads and their ability to move through strategically positioned slots allows them to not contribute disadvantageously to the stiffness of the flexure arm. Ohwe et al. (U.S. Pat. No. 6,560,073) also teaches a flexure with a transverse cross beam extending from a pair of cross beams formed along opposite sides of the flexure. A central portion of the flexure, on which the slider is mounted, is supported by the transverse cross-beam. This flexure is very similar to that of Ueda, cited above.
The surface of the disk is not perfectly flat. It has texture, waviness and asperity. The slider fly height is also not constant. It is subject to variations due to ambient air pressure, shocks, wind excitations, disk topography and track accessing. Therefore, intermittent contact between the slider and the disk surface (the “head disk interface” interaction or HDI) does occasionally occur. The frequency and the intensity of such HDI increases with decreasing nominal fly height. Shimizu et al. (U.S. Pat. No. 5,659,448) teach a method of reducing the variations in slider fly height and of thereby reducing slider positioning error, by a method of affixing the slider to the gimbal using a small spacer, thereby reducing the amount of warpage in the gimbal that is transmitted by the motion of the slider. It has also been noted in the prior art that assembly errors also lead to poor slider performance and to general unreliability of the disk assembly. Ohwe et al. (U.S. Patent Application Publication No. U.S. 2005/0083610 A1) teach a magnetic head supporting system in which a gimbal assembly includes pairs of supporting beams, producing a more stable fly height.
HDI events cause wear to both the slider and the disk surface, creating debris and eventually leading to catastrophic failure, the “head crash.” While this is clearly undesirable, advances in slider and disk surface tribology (study of frictional interactions) have significantly delayed the occurrence of head crashes. The slider-disk interface is now sufficiently durable to permit relatively intense HDI. In this regard, Boutaghou et al. (U.S. Pat. No. 6,392,842) teach the fabrication of a low friction surface slider that is capable of operation at ultra-low flying heights. In accord with such developments relating to improvements in the slider itself, the nominal fly height is being reduced in pursuit of high data recording density. However, this attempt is hampered by a new challenge, fly height modulation. Unlike the catastrophic head crashes that reduce the useful lifetimes of disk drives, excessive modulation renders a disk drive inoperable as soon as it is built. It is commonly understood that extensive HDI transfers kinetic energy from the rapidly rotating disk to a nominally stationary slider. Vertical vibration of the slider affects fly height and, therefore, affects the ability to record on and retrieve data from an intended track. Radial vibration of the slider causes the read/write transducer to record data on and retrieve data from tracks that are adjacent to the target track, creating irrecoverable (hard) and recoverable (soft) errors, respectively.
Much effort has been given by the magnetic recording industry to the improvement of slider ABS design, in order to improve the air bearing stiffness and damping. The prevailing theory is that slider movement in response to HDI should be minimized. By stiffening the air bearing interaction, the slider can better maintain its fly height during HDI. By improving damping, the slider fly height can be better recovered after each HDI event. This approach has been unsuccessful, however, in reducing fly height modulation at near-contact conditions.
The present inventors have discovered that fly height modulation associated with HDI is strongly influenced by high frequency (above 50 kHz) vibrations of the flexure. Specifically, this is because the flexure affects kinetic energy transfer between the disk and HGA during HDI. The flexure can store a significant amount of kinetic energy, thereby reducing the damping coefficient of the air bearing resonance. In view of this connection, there is a need for a flexure that transfers and stores minimal amounts of kinetic energy during and after HDI, therefore minimizing the fly height modulation at near-contact conditions.
The present invention teaches a flexure system design that has distinct advantages over designs within the prior art cited above. The objects of the present invention and the means of achieving those objects will be presented below.